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.Dd October 10, 2020
.Dt G_ATTACH 9
.Os
.Sh NAME
.Nm g_attach ,
.Nm g_detach
.Nd "attach/detach GEOM consumers to/from providers"
.Sh SYNOPSIS
.In geom/geom.h
.Ft int
.Fn g_attach "struct g_consumer *cp" "struct g_provider *pp"
.Ft void
.Fn g_detach "struct g_consumer *cp"
.Sh DESCRIPTION
The
.Fn g_attach
function attaches given consumer
.Fa cp
to given provider
.Fa pp ,
thus establishing a communication channel between the consumer and the
provider that allows to change access counts and perform I/O operations.
.Pp
The
.Fn g_detach
function detaches given consumer
.Fa cp
from its corresponding provider, tearing down the communication channel
between them.
.Sh RESTRICTIONS/CONDITIONS
.Fn g_attach :
.Bl -item -offset indent
.It
The consumer must not be attached to a provider.
.It
The operation must not create a topology loop.
.It
The topology lock has to be held.
.El
.Pp
.Fn g_detach :
.Bl -item -offset indent
.It
The consumer has to be attached.
.It
The access count has to be 0.
.It
There must be no active requests.
.It
The topology lock has to be held.
.El
.Sh RETURN VALUES
The
.Fn g_attach
function returns 0 if successful; otherwise an error code is returned.
.Sh EXAMPLES
Create a consumer, attach it to a given provider, gain read access and clean up.
.Bd -literal -offset indent
void
some_function(struct g_geom *mygeom, struct g_provider *pp)
{
	struct g_consumer *cp;

	g_topology_assert();

	/* Create new consumer on 'mygeom' geom. */
	cp = g_new_consumer(mygeom);
	/* Attach newly created consumer to given provider. */
	if (g_attach(cp, pp) != 0) {
		g_destroy_consumer(cp);
		return;
	}
	/* Open provider for reading through our consumer. */
	if (g_access(cp, 1, 0, 0) != 0) {
		g_detach(cp);
		g_destroy_consumer(cp);
		return;
	}

	g_topology_unlock();
	/*
	 * Read data from provider.
	 */
	g_topology_lock();

	/* Disconnect from provider (release access count). */
	g_access(cp, -1, 0, 0);
	/* Detach from provider. */
	g_detach(cp);
	/* Destroy consumer. */
	g_destroy_consumer(cp);
}
.Ed
.Sh ERRORS
Possible errors:
.Bl -tag -width Er
.It Bq Er ELOOP
The operation creates a topology loop.
.It Bq Er ENXIO
Provider got orphaned.
.El
.Sh SEE ALSO
.Xr geom 4 ,
.Xr DECLARE_GEOM_CLASS 9 ,
.Xr g_access 9 ,
.Xr g_bio 9 ,
.Xr g_consumer 9 ,
.Xr g_data 9 ,
.Xr g_event 9 ,
.Xr g_geom 9 ,
.Xr g_provider 9 ,
.Xr g_provider_by_name 9 ,
.Xr g_wither_geom 9
.Sh AUTHORS
.An -nosplit
This manual page was written by
.An Pawel Jakub Dawidek Aq Mt pjd@FreeBSD.org .
